The present invention relates to a method and apparatus for utilizing a direct writing electron beam system as a measurement tool for measuring overlay error. More particularly, the invention comprises a novel method of measuring overlay error between a first pattern and a second pattern which are deposited upon a wafer utilized in semiconductor manufacturing.
In fabricating semiconductor wafers it is necessary to fabricate multiple layers upon the wafer in order to produce specific semiconductor devices and interconnections to these devices. This entails a requirement for accurately aligning the base of a semiconductor device, for example, in a first layer with an emitter for the same semiconductor device, fabricated as part of a second layer or processing step. The non perfect alignment which occurs between a feature in the first layer and a feature in a subsequently applied layer is referred to in the art as overlay error.
Various techniques have been utilized in the prior art to accurately position multiple layers with satisfactory alignment. Heretofore, none of the methods entailed the use of an electron beam system which was also utilized in the manufacturing process as a direct writing instrument. Accordingly, the present invention involving the use of an electron beam system as an overlay measurement instrument is believed to be novel. It enables large volumes of overlay data to be automatically and accurately collected, eliminating operator errors that would result from a more conventional means of data gathering. Furthermore, it can result in further cost efficiencies since it may not require utilization of a second instrumentation means to control overlay alignment but, in the preferred embodiment, uses the fabrication means with no hardware modifications.
The following patents, all assigned to International Business Machines Corporation, provide background information and are incorporated herein by reference.
U.S. Pat. No. 3,644,700 for "Method And Apparatus For Controlling An Electron Beam" by Kruppa.
U.S. Pat. No. 3,866,013 for "Method And Apparatus For Controlling Movable Means Such As An Electron Beam" by Ryan.
U.S. Pat. No. 3,875,415, for "Method And Apparatus For Detecting A Registration Mark On A Target Such As A Semiconductor Wafer" by Woodard.
U.S. Pat. No. 3,901,814 for "Method And Apparatus For Detecting A Registration Mark On A Target Such As A Semiconductor Wafer" by Davis et al.
U.S. Pat. No. 3,900,736 to Michail et al. for "Method And Apparatus For Positioning A Beam Of Charged Particles".
U.S. Pat. No. 4,056,730 to Davis et al., for "Apparatus For Detecting Registration Marks On A Target Such As A Semiconductor Wafer".
Of the prior art patents mentioned above, U.S. Pat. No. 4,056,730 is believed to be of significant value in terms of providing background for this invention. The aforesaid patent describes apparatus for detecting registration marks on a semiconductor wafer including means for operating upon signals picked-up by diode detectors when electrons are backscattered from the target. The apparatus disclosed therein further includes means for differentiating the backscatter signal, and if more than one signal is used, means for summing the signals just before or just after differentiating. The purpose of the Davis et al. patent is to provide a technique for accurately determining when the electron beam spot of the system scans over a particular registration mark or indicia.